The internal structure of objects that are light impenetrable, running the gamut from the human body to turbine blades and pipe-line welds, can be non-destructively examined, by impinging X-radiation uniformly on the object and recording the image pattern of X-radiation that emerges from the object.
Radiographic elements containing silver halide grains are commonly used for recording X-radiation images. In the indirect approach, which accounts for the majority of medical diagnostic imaging, the X-radiation is absorbed primarily by a fluorescent intensifying screen. Phosphor particles in the screen absorb X-radiation and emit light that is in turn absorbed by the silver halide grains in a radiographic element to form a developable latent image. In this approach the silver halide grains are spectrally sensitized so that the peak sensitivity of the grains matches the principal emission peak of the phosphor particles.
In the direct approach, absorption of the X-radiation by the silver halide grains is relied upon to produce a latent image that can then be converted to a viewable form by processing (e.g., development and fixing).
In both direct and indirect radiographic imaging the imaging sensitivity (i.e., speed) is increased by chemically sensitizing the silver halide grains. Sulfur and gold sensitizers, singly and, most commonly, together are employed. Common chemical sensitizers for silver halide grains are summarized in Research Disclosure, Vol. 389, September 1996, Item 38957, IV. Chemical sensitization. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
About 10 times more X-radiation is required to produce a latent image by direct X-radiation exposure than by indirect X-radiation exposure. In medical diagnostic imaging direct X-radiation imaging is confined to applications where only low levels of X-radiation exposure are required and use of indirect X-radiation imaging is inconvenient. For example, in dental diagnostic imaging a small piece of X-ray film, commonly referred to as a "chip", mounted in a sealed opaque package, is placed in the patient's mouth during X-ray exposure. The oral location of the film during exposure renders the use of intensifying screens difficult and, when the small area of exposure and therefore small benefit to be gained is taken into account, impractical.
X-radiation imaging of inanimate objects, commonly labeled industrial X-ray imaging, usually does not have the same level of object sensitivity to X-radiation as medical diagnostic imaging. Hence, direct X-ray imaging of objects such as turbine blades and pipe line welds is quite common.
Both direct and indirect imaging radiographic elements that rely on silver halide for latent image formation are light sensitive. Since indirect silver halide radiographic elements are intended to respond to light emitted by one or two intensifying screens and are usually spectrally sensitized, it is readily apparent that these elements must be sensitive also to ambient light. Although direct radiographic elements are intended to record only X-radiation and cannot benefit from spectral sensitization, the silver halide grains have native sensitivity extending from the near ultraviolet into the visible spectrum. The hydrophilic colloid, such as gelatin, suspending the silver halide grains provides radiation exposure protection for wavelengths up to 320 nm, but the native sensitivity of the grains to wavelengths longer than 320 nm causes objectionably increased minimum density levels when inadvertent exposure to ambient light occurs.
Medical X-ray clinics have little difficulty with protecting radiographic elements from light exposure prior to and during processing. The radiographic film is protected from room light by enclosure in a cassette during exposure and handling, and standard equipment exists for the dark loading of the film into the cassette and the dark removal of the film from the cassette for processing in a light sealed rapid access processor.
However, for many other users the necessity of protecting the radiographic element from ambient light until processing is completed is burdensome. For example, in dental diagnostic imaging, following exposure, the dentist or dental technician must leave the patient and retreat to a separate room equipped with safelights to remove the dental film chip from its opaque package and complete processing to a viewable image. Leaving the patient is an inconvenience and maintaining a separate room equipped with safelights is a major expense.
In industrial radiography the direct X-ray film is exposed in a light-tight container. The light-tight container is removed and processing is completed under safelights. Often in industrial radiography it is not feasible to bring the object to the X-ray examination equipment. Thus, for imaging a service truck providing a dark room containing a processor is required.
Attempts to integrate light exposure protection into direct X-ray films are known, but have not been accepted. For example, Murray U.S. Pat. No. 2,379,373 discloses overcoating the emulsion layers of direct X-ray films with carbon black in a casein and gelatin vehicle that can be removed during processing. Boucher U.S. Pat. No. 2,542,304 discloses laminating strippable opaque layers over the surface of the emulsion layers in direct X-ray films. Little, if any, commercial use of these approaches has occurred.